Electric fish weak on charge, strong on adaptation

An
electric eel can generate enough current to stun its prey, just like a
Taser. Weakly electric fish can also generate electricity, but not
enough to do any harm.

"Weakly
electric fish are unique in that they produce and detect electric
fields. They use these electric fields in social communication and to
detect objects," explains Johns Hopkins University neuroethologist Eric
Fortune.

Fortune
has traveled to Ecuador to study weakly electric knifefish in their
native habitat, even placing acoustical instruments underwater so he
could listen to and record their electrical hums.

Back
at Johns Hopkins University, research collaborator and mechanical
engineer Noah Cowan and the rest of the team use Fortune's field data to
help with their observations and experiments in the lab. With support
from the National Science Foundation (NSF), they are studying the
knifefish to learn more about how the brains of animals work to control
their behavior.

"We
see how they interact in the wild and then we create very controlled
experiments in the lab that allow us to probe specific scientific
questions. Researchers want to better understand how these fish use
their electric field as a sixth sense, not only to communicate with each
other, but to navigate their surroundings and find their next meal,"
explains Cowan.

"There's
a small organ in the tail of the weakly electric fish that generates an
electric field that envelops the entire animal," continues Cowan. When
an object passes through the field, the fish has receptors on its skin
to detect the object. "There are little voltage sensors all over the
surface of the skin and as an object comes by, the voltage changes and
it says, 'Ah-ha, lunch,' or it says, 'I'm going to be lunch,' and it
swims away."

Each
knifefish can generate its own frequency that, in some cases, can
change when another knifefish comes near. One reason may be to avoid
jamming each others' signals, and another may be to communicate. "When
two fish come near each other, their two pitches begin to interact much
like two singers' pitches would interact," says Cowan.

The
researchers also want to study what happens when more than two
interact. When their electric fields overlap, does it heighten or reduce
their ability to detect predators and prey? If grouping together is a
benefit, a chorus of hums might prove more beneficial for these fish
than going solo. "The fact that there are multiple frequencies present
at the same time and they're moving around together is a complicated
puzzle that we haven't yet figured out," says Cowan.

"The
power of this research lies in the combination of field studies with
analytical laboratory experiments. Few researchers so clearly excel in
both domains," says Elizabeth Cropper, a program director in NSF's
Biological Sciences Directorate, which funded the research.

Another
piece of the research puzzle is learning when the fish relies on its
electro-sense over its other five senses. When the lights go out and
it's hard for the fish to see, they seem to rely on it more for
navigation. Their electro-sense is a lot better than vision in places
where the water gets turbid. The electric field goes right through it.
Knifefish are also agile swimmers propelled by long ribbon fins. "They
can swim forward, backward, and rotate rapidly," notes Cowan.

Their
sixth sense and enhanced agility are making them good role models in
the development of small submersible robots. Malcolm MacIver, a
mechanical and biomedical engineer at Northwestern University, and his
team are developing a nimble robot that can swim backwards and forwards.
It may one day be able to use a similar "sixth sense" to monitor the
health of coral reefs, or to navigate the dark, murky waters of an oil
spill.